Electrophotographic copying machines show a yearly increase in copying speed, and machines on which various sizes of paper can be copied have been developed. With such development, high-sensitive and long-lived photoreceptors have been desired so as to be able to comply therewith.
Recently, many function separation type electrophotographic photoreceptors, in which the photoreceptor functions are assigned to a plurality of members, have been proposed to improve electrophotographic characteristics such as charge keeping characteristics, repetition stability, light response, spectral characteristics, and mechanical strength.
These electrophotographic photoreceptors are known to have the following disadvantages:
(1) They are short of repetition stability of development contrast and environmental stability;
(2) Image defects called white spots, black spots, roughening, and pin holes are liable to be developed; and
(3) The adhesive strength between the substrates and the photoconductive layers is low, and therefore, the photoreceptive layers are separated at the time of use to result in insufficient durability.
In order to solve these problems, attempts have been made to provide resin layers between the substrates and the photoconductive layers. Examples of the resins which are known to be used include polyparaxylene, casein, polyvinyl alcohol, phenyl resins, polyvinyl acetal resins, melamine resins, nitrocellulose, ethylene-acrylic acid copolymers, polyamides (such as nylon 6, nylon 66, nylon 610, copolymerized nylon and alkoxymethylated nylon), polyurethanes, gelatin, polyvinylpyrrolidone, polyvinylpyridine, and polyvinylmethyl ether.
Further, it has also been variously proposed to form intermediate layers using organic zirconium compounds such as zirconium chelate compounds and zirconium alkoxides and silane coupling agents, as described, e.g., in JP-A-59-223439 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-61-94057, and JP-A-62-273549.
When the resin layer is provided as an underlayer, it mainly comprises a resin having a relatively large amount of polar groups, thereby controlling the volume resistivity to the extent that the electrophotographic characteristics are not deteriorated. However, the volume resistivity of the resin depends on the ion conductivity in many cases, so that it is significantly affected by temperature and humidity. Accordingly, when the photoreceptor is placed under the circumstances of low temperature and humidity or high temperature and humidity, the resin layer is markedly increased in resistivity to cause deterioration of the electrophotographic characteristics of the photoconductive layer, or the resin layer is significantly lowered in resistivity to cause the desired function of the resin layer to disappear.
According to the known resin layers, therefore, the drawbacks of the photoreceptors are only partly improved, or the effect is reduced by half, with the consideration of environmental characteristics, etc. Accordingly, they are technically very insufficient.
On the other hand, use of an organic zirconium compounds or a silane coupling agents fairly improves the above-described problems. However, the problem arises that a decrease in development contrast is induced associated with an increase in residual potential. Furthermore, for example, use of the zirconium chelate compounds causes insufficient curing reactivity, and use of the zirconium alkoxides or the silane coupling agents results in aging instability of coating solutions due to hydrolysis or aging instability in forming coated films. Thus, the organic zirconium compounds and the silane coupling agents are not necessarily satisfactory because of their drawbacks in preparing the underlayers.
As described above, the underlayers proposed in the prior art are not sufficient yet to remove various problems of the electrophotographic photoreceptors, which causes unsatisfactory characteristics of the photoreceptors.